The present invention relates to a structured packing having particular application to a method of separating air in which the packing is formed of a plurality of corrugated sheets and a plurality of flat, planar members alternating with and located between the corrugated sheets to inhibit vapor turbulence . The planar members may be substantially the same size as the corrugated sheets or smaller in length than the corrugated sheets. When substantially the same length, the corrugated sheets and the flat, planar members have perforations sized to inhibit vapor and liquid flows while allowing for transverse pressure equalization through the structured packing. When the planar members are smaller in length one or more planar members may be present between the corrugated sheets.
Structured packing has found wide spread use in a variety of distillations including those involved in the separation of air into its component parts. Distillations are conducted within distillation columns filled with mass transfer elements to bring ascending vapor phases into intimate contact with descending liquid phases of mixtures to be separated. As the ascending phase rises and contacts the descending liquid phase, it becomes evermore enriched in the more volatile components of the mixture to be separated. At the same time the descending liquid phase becomes ever more concentrated in the less volatile components of the mixture to be separated. In such fashion, systems of distillation columns can be used to separate various mixture components. For instance, in case of air separation, nitrogen is separated from oxygen in a double distillation column unit. Argon is then separated from oxygen in an argon column that is attached to a lower pressure column of such double distillation column unit.
Structured packings are widely used as mass transfer elements within distillation columns due to their low pressure drop characteristics. Structured packings generally include corrugated sheets of material in which the sheets are placed in a side by side, relationship with the corrugations of adjacent sheets criss-crossing one another. In use, the liquid phase of the mixture to be separated is distributed to the top of the packing and spreads out throughout the packing as a descending film. The vapor phase of such mixture rises through the corrugations contacting the liquid film as it descends.
There have been many attempts in the prior art to increase the efficiency of structured packings, that is, to decrease the height of packing equal to a theoretical plate. Obviously, the lower the height, the more efficient the packing. At the same time, structured packing with a low HETP inherently has an increased pressure drop over less efficient packings. One such structured packing is disclosed in U.S. Pat. No. 4,597,916 in which the corrugated sheets are separated from one another by flat, perforated sheets that extend throughout the packing. It is believed that the flat perforated sheets of this prior packing increase efficiency by both providing additional interfacial area for vapor-liquid contact and by increasing turbulence in the vapor flow and therefore the degree of mixing between vapor and liquid phases. Unlike the present invention, transverse mixing is also increased by perforations that are specifically designed and sized to promote liquid and vapor flow in a transverse direction of the packing.
As will be discussed, the present invention is a structured packing that, unlike the prior art, is optimized not for efficiency, but rather, for smooth vapor flow. Through such optimization, it is possible to increase the capacity of the packing and therefore, use such packing in a more efficient cost effective manner.
However, the present packing inhibits transverse liquid and vapor flow thereby decreasing the degree of mixing. The decreased mixing is a significant factor that results in increased packing capacity.
The present invention provides a structured packing comprising a plurality of corrugated sheets and a plurality of flat, planar members alternating with and located between the sheets to inhibit turbulence in vapor ascending through said structured packing. The planar members may be sized with lengths and widths equal to those of the corrugated sheets or may be smaller in length than the corrugated sheets. The plurality of planar members are positioned so that at least one horizontal edge of the planar members and the corrugated sheets are situated proximal to one another as viewed when the structured packing is in use. As used herein, length and width refer to the dimensions of the structured packing, planar members and corrugated sheets also as viewed when the structured packing is in use. The length dimension is measured parallel to the longitudinal axis of the distillation column in which the structured packing is used.
In one embodiment of this invention where the planar members are substantially the same length as the corrugated sheets, each of the planar members and the corrugated sheets have perforations sized to inhibit transverse liquid and vapor flow while allowing pressure equalization. In another embodiment, one or more planar members or strips can be located between the corrugated sheets and may be perforated or non-perforated.
An example of this embodiment includes pairs of the planar members located between the corrugated sheets and spaced apart from one another so that the uppermost and the lowermost horizontal edge of the planar members and the corrugated sheets are aligned.
In those embodiments, having perforations each of the perforations have a diameter in a range of between about 5% and about 40% of the channel width of the corrugations in the corrugated sheets as measured between adjacent peaks or troughs of the corrugations. This diameter can be between about 5% and about 20% of the channel width. Preferably the diameter is about 10% of the channel width of corrugations. Furthermore, the perforations can constitute an open area of the planar members in a range of between about 5% and about 20% of a total area of the planar members. Such open area of the planar members can be between about 7% and about 15% of the total area. Preferably, the open area of the planar members is about 10% of the total area.
The length of the planar members may be smaller than the length of the corrugated sheets. In this embodiment, it is preferred that the length be less than about one-third, more preferably less than one-fifth, the length of the corrugated sheets. When the length of the planar members are less than about one-third of the length of the corrugated sheets, sufficient pressure equalization can normally occur and perforations are generally not required.
It has been found that a structured packing designed in the manner set forth above functions with a slightly higher HETP than structured packings of the prior art. This is surprising considering the fact that the packing with the intermediate planar members has a greater surface area than similar packing not incorporating such planar members. A further unexpected feature is that all packings of each embodiment of the present invention flood at higher vapor rates. There are various criteria that are used to describe the flooding condition, for instance, excessive pressure drop. In all cases if HETP is plotted against F-Factor (where F-Factor is a product of the superficial vapor velocity and the square root of the vapor density) flooding is evidenced by a rapid rise of the slope of the curve. Such a rise in HETP is indicative of the vapor supporting the descending liquid thereby choking the column and disrupting the separation. This increase in the flooding point allows higher flow rates through the column and therefore for a given volume of packing, greater production. This allows for thinner columns using less packings or columns that can handle a greater throughput. The reason for such operation is that the planar member and opening design of the present invention are believed to inhibit turbulence in the vapor now ascending through the structured packing.